Corrosion inhibiting organic coatings have been applied for many years to metal in coil form or sheet form prior to forming into articles. One of the problems encountered in using this pre-coated metal is that, if these articles are to be assembled, they must be joined mechanically since organic coatings are generally not weldable as a practical matter. In some applications, an organic coating with a powdered aluminum corrosion inhibitor has some slight degree of weldability which requires special techniques and equipment to be able to pass sufficient current to weld the substrate. Even with this coating, when welding can be achieved, the welded spot or area is not protected from corrosion by the coating.
The only organic coating known to be weldable without resorting to special techniques and equipment contains powdered zinc as a corrosion inhibitor. This type of coating provides cathodic protection against galvanic corrosion of the metal substrate. However, when the coating is in use on a metal substrate, it forms corrosion or oxidation by-products, such as zinc hydroxide, zinc chloride and zinc sulfide. These by-products make this coating unsuitable for certain applications, such as the interior surfaces of vessels for petrol chemicals such as brake fluid, alcohol, gasoline and diesel fuel. These by-products are precipitates which become suspended in the brake fluid, alcohol, gasoline or fuel oil and will contaminate and clog brake cylinders, carburetors and fuel injectors and produce deleterious effects in catalytic converters for controlling the emissions of automotive and truck engines.
U S. Pat. No. 4,391,855 discloses for a metal substrate an effective corrosion inhibiting and weldable organic coating containing a phenoxy resin and powdered zinc. This patent also discloses an organic coating having a phenoxy resin and powdered aluminum, cadmium and/or stainless steel, which in most applications cannot be welded, and in others can only be welded by special techniques and equipment.